Constraints on fault and lithosphere rheology from the coseismic slip and postseismic afterslip of the 2006 Mw7.0 Mozambique earthquake

The 2006 M_w7.0 Mozambique (Machaze) normal-faulting earthquake ruptured an unusually steeply dipping fault plane (∼75°). The amount of slip in the earthquake decreased from depths of ∼10 km toward the surface, and this shallow slip deficit was at least partly recovered by postseismic afterslip on the shallow part of the fault plane. An adjacent normal fault segment slipped postseismically (and possibly also co-seismically) at shallow depths with a large strike-slip component, in response to the stresses generated by slip on the main earthquake fault plane. Our observations suggest that the fault zone behaves in a stick-slip manner in the crystalline basement, and that where it cuts the sedimentary layer the coseismic rupture was partially arrested and there was significant postseismic creep. We discuss the effects of such behavior on the large-scale tectonics of continental regions, and on the assessment of seismic hazard on similar fault systems. The steep dip of the fault suggests the re-activation of a preexisting structure with a coefficient of friction at least ∼25-45% lower than that on optimally oriented planes, and analysis of the deformation following an aftershock indicates that the value of the parameter `a' that describes the rate-dependence of fault friction lies in the range 1 × 10^-3-2 × 10^-2. The lack of long-wavelength postseismic relaxation suggests viscosities in the ductile lithosphere of greater than ∼2 × 10¹⁹ Pa s, and an examination of the tectonic geomorphology in the region identifies ways in which similar fault systems can be identified before they rupture in future earthquakes.